A number of radiolabeled 2′-deoxy-2′-fluoro-5-substituted-1-β-D-arabinofuranosyl-uracil and -cytosine derivatives have been recognized as efficient probes for imaging tumor proliferative activity and HSV1-tk reporter gene expression with positron emission tomography (PET). Among these, 2′-deoxy-2′-[18F]fluoro-5-methyl-1-β-D-arabinofuranosyl-uracil ([18F]-FMAU), 2′-deoxy-2′-fluoro-5-[11C]methyl-1-β-D-arabinofuranosyl-uracil ([11C]-FMAU) and 2′-deoxy-2′-[18F]fluoro-5-bromo-1-β-D-arabinofuranosyl-uracil ([18F]-FBAU) are markers for DNA synthesis through phosphorylation by human and other mammalian nucleoside kinases including thymidine kinase TK1 and TK2, and FMAU is currently undergoing clinical studies in multiple centers for imaging tumor proliferation in a variety of cancer types and DNA synthesis. The other derivatives, such as 2′-deoxy-2′-[18F]-fluoro-5-iodo-1-β-D-arabinofuranosyluracil ([18F]-FIAU), 2′-deoxy-2′-[18F]fluoro-5-fluoro-1-β-D-arabinofuranosyl-uracil ([18F]-FFAU) and 2′-deoxy-2′-[18F]-fluoro-5-chloro-1-β-D-arabinofuranosyl-uracil ([18F]-FCAU) are excellent substrates for the viral kinases such as herpes simplex virus (HSV) type 1 and 2, and FIAU is also a substrate for hepatitis B-virus and Epstein B virus (EBV) thymidine kinase. These 2′-fluoro-5-substituted arabinosyluracil derivatives were synthesized and evaluated earlier as antiviral agents. Recently, 18F-1-(2′-deoxy-2′-fluoro-arabinofuranosyl)cytosine (18F-FAC), 2′-deoxy-2′-18F-fluoro-5-methyl-beta-L-arabinofuranosylcytosine (18F-FMAC) and their analogs have been shown to be potential PET tracer for cancer disease, autoimmunity inflammation, and bone marrow transplant. The first radiochemical synthesis of FMAU with PET isotope ([11C]) was reported by the applicants. However, due to the short half-life of [11C](t1/2=20 min), the applicants developed the radiosynthesis of [18F]-labeled FMAU and other 5-substituted thymidine analogues. After this synthesis was disclosed, another group of investigators also reported the [18F]-labeled synthesis of these pyrimidine nucleoside analogues.
The radiosynthesis of F-18 FMAU (Scheme 1) involves radiofluorination of 2-trifluoromethane-sulfonyl-1,3,5-tri-O-benzoyl ribofuranose to 2-[18F]-fluoro-1,3,5-tri-Obenzoyl arabinofuranose derivative, followed by conversion to 1-bromo-2-[18]-fluoro-1,3,5-tri-O-benzoyl derivative, then coupling of the 1-bromo-2-[18]-fluoro-2,3,-di-O-benzoylarabinofuranose with 2,4-bis-trimethylsilyluracil derivatives. Finally, hydrolysis of the protecting groups from the sugar moiety and HPLC purification produces the desired products. Specific reaction conditions and reagents used are detailed in Scheme 1.
An ideal radiosynthesis procedure involves a single step radiolabeling of a precursor compound, followed by hydrolysis of protecting groups, if necessary, and purification of the crude mixture. However, such an ideal method has not been successful when applied to the radiolabeling of 2′-fluoroarabinosubstituted pyrimidine nucleosides. Multiple steps are required after radiolabeling of the sugar moiety.

Although the applicants and other researchers in the field have demonstrated these reactions are very reliable and reproducible, the complexity of this method often requires significant modification of existing commercial automated modules, accompanied by frequent production failures. In order to find an efficient fully automated Current Good Manufacturing Practice-compliant (cGMP-compliant) radiosynthesis system for the production of these probes, the applicants have been optimizing the reaction conditions to reduce synthetic time and simplify reaction conditions. Recently, the applicants reported the use of Friedel-Crafts catalysts for a synthesis of 18F-FMAU, which also includes a significantly simplified one-pot reaction condition (see below). However, a need exists for the fully automated synthesis of [18F]-FMAU using one pot reaction conditions. According to certain embodiments, the method is compatible with most commercially available modules typically used for production of cGMP compliant radiotracers for clinical applications.
